Low compression screw

ABSTRACT

A method of injection molding or extruding a polymer composition having a predetermined bulk density associated with particles, granules or pellets of the polymer composition and a predetermined melt density when the polymer composition is fully melted and compressed. The method of the invention employs a screw having a volume compression ratio that is greater than or equal to the ratio of the predetermined melt density to the predetermined bulk density of the polymer composition and up to 1.25× the ratio of the predetermined melt density to the predetermined bulk density of the polymer composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. application Ser. No.10/209,253, filed Jul. 30, 2002, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 09/585172,filed Jun. 1, 2000, now abandoned, which is a continuation-in-part ofU.S. patent application Ser. No. 09/334,331, filed Jun. 16, 1999, nowabandoned, which is a continuation-in-part of U.S. patent applicationSer. No. 09/283,516, filed Apr. 1, 1999, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 09/073,281,filed May 6, 1998, now abandoned.

BACKGROUND

1. Field of the Invention

This invention relates to the field of screws that are used, forexample, to melt or soften polymer, such as in a machine for injectionmolding polymer or a machine for extruding polymer.

2. Description of the Related Art

The use of screws to injection mold or extrude polymer is well known.Turning to FIG. 1, there is shown a conventional or standard screw 11for use in injection molding which includes three zones: a feeding zone13, a compression or transition zone 15 and a metering zone 17. Screw 11is housed in a hollow cylindrical barrel 19 having a constant innerdiameter and preferably a smooth inner surface. Polymer resin, which maybe in any form such as pellets, granules, flakes or powder, is fedthrough an opening 21 in barrel 19 into feeding zone 13 where screw 11turns to pack and then push the pellets into compression zone 15. Thepellets are melted in compression zone 15 and then pushed to meteringzone 17 where the molten material is homogenized. Afterwards thehomogenized melt is either injection molded or processed further.

Screw 11 has a screw shaft 23 having a thread 25 spirally positionedabout shaft 23 to form flights 25. Flights 25 are characterized by theirdepth, which is the height of flight 25 above shaft 23 and by theirpitch, which is the length P of the distance between two adjacentflights 25 plus one flight width. The outside diameter OD of a screw 11includes the depth of a flight 25 above and below shaft 23, whereas theroot diameter RD of screw 11 is the diameter of shaft 23 only, withoutincluding the depth of flights 25. Conventionally flights 25 in a screw11 have the same pitch in each of feeding zone 13, compression zone 15and metering zone 17, but have a changing depth from zone to zone.Specifically, flights 25 have a constant depth x in feeding zone 13, aconstant depth y in metering zone 17 where y<x, and a graduallydecreasing depth of x to y in compression zone 15.

Screws are often characterized by their compression ratio, which is aratio that is used to quantify the amount the screw compresses orsqueezes the resin. The concept behind the compression ratio is todivide the volume of a flight in the feed section by the volume of aflight in the metering section, but the actual standard that is used isa simplified method based on the following equation:${{Compression}\quad{ratio}} = \frac{{depth}\quad{of}\quad{flight}\quad{in}\quad{feeding}\quad{zone}}{{depth}\quad{of}\quad{flight}\quad{in}\quad{metering}\quad{zone}}$

This compression ratio is referred to as the depth compression ratio.High compression screws, which are usually used for crystalline orsemi-crystalline materials, such as polymers, have compression ratios ofgreater than about 2.5. Standard compression screws, which are usuallyused for amorphous materials, have compression ratios of from about 1.8to about 2.5, more commonly 2.2.

Various problems with high compression screws include: overheatingcaused by compression that is too high or is uncontrolled; “bridging”,which is when the polymer melt turns with the screw and is not pushedforward; and screw deposit which builds up in the compression andmetering zones. These problems limit the maximum screw rotation speedand by consequence the output of molten material. In an attempt toovercome these problems some users switch to standard screws, but thedepth of the flight in the metering zone of a standard screw is too highto give good melt homogeneity under some conditions, especially withcrystalline materials.

Many attempts have been made to improve the performance of screws. U.S.Pat. No. 4,129,386, discloses an extrusion device which has a groovedbarrel in combination with a screw having a helix angle or pitch D inthe feed zone that constantly increases through a transition zone to ahelix angle F in the metering zone. The feed zone has a constant flightheight G, the metering zone has a constant flight I, and the ttransition zone has a constantly decreasing flight height from feed zoneflight height G to metering zone flight height I. This screw designsuffers from problems of overfeeding of the material to be extruded, andrequires a grooved barrel in order to prevent buildup of excessivepressure gradients along the screw.

What is needed, therefore, is a screw which will produce a homogeneousmelt without the problems associated with screws having a highcompression ratio.

SUMMARY OF THE INVENTION

The present invention relates to a screw for use in, for example, aninjection molding machine or an extruder. The screw includes a screwshaft having a thread spirally positioned about the screw shaft so as toform a plurality of flights which are divided into three zones: afeeding zone, a compression zone and a metering zone.

The depth, width and pitch of the flights of the screw are designedbased upon the material to be used in the screw so that the differencein the ratio of the actual volumetric flow to the theoretical volumetricdrag flow of material in the feeding zone and the ratio of the actualvolumetric flow to the theoretical volumetric drag flow of material inthe metering zone is less than 0.2, preferably less than 0.1, and morepreferably less than 0.05. In a preferred embodiment the ratio of theactual volumetric flow to the theoretical volumetric drag flow ofmaterial in the feeding zone and/or the ratio of the actual volumetricflow to the theoretical volumetric drag flow of material in the meteringzone is from about 0.8 to 1.0.

This design results in a screw which has a balanced mass flow throughoutthe screw, and thus a constant pressure gain along the screw withoutpressure peaks.

An example of a screw that has the desired difference in the ratio ofthe actual volumetric flow to the theoretical volumetric drag flow ofmaterial in the feeding zone and the ratio of the actual volumetric flowto the theoretical volumetric drag flow of material in the metering zoneis a screw in which the pitch of at least a portion of the flights inthe metering zone is greater than the pitch of at least a portion of theflights in the feeding zone; the pitch of at least a portion of theflights in the feeding zone is less than the outside diameter of thescrew; the pitch of at least a portion of the flights in the meteringzone is greater than the outside diameter of the screw; the pitch of atleast a portion of the flights increases through the compression zone;and the depth of at least a portion of the flights decreases through thecompression zone moving from nearer the feeding zone to nearer themetering zone.

The invention also relates to a method for designing a screw such thateach flight of the screw has the same mass of material, taking intoaccount the physical state of the material in such flight, either asunmelted pellets or granules of material, partially melted pellets orgranules of material, or completely melted material. To obtain thisconstant mass, the volume of each flight of the screw is designed insuch a way that the compression ratio in volume of a flight in thefeeding zone to the volume of a flight in the metering zone equals theratio of the melt density/bulk density of the material to be used in thescrew. It is preferred that the volume compression ratio of the screw isincreased above the melt density/bulk density ratio by a factor of up toabout 25% to be sure that there is a continuous homogenous feeding ofthe melt in the screw. The physical design of the screw is then only aconsequence of this method of design.

The inventive screw allows a higher screw rotation speed, has a higherthroughput, and decreases the injection molding cycle time compared toconventional screws.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view in elevation of a standard screw; and

FIG. 2 is a side view in elevation of a screw made in accordance withthis invention.

DETAILED DESCRIPTION

The present invention relates to a screw for use in, for example, aninjection molding machine or an extruder. The screw includes a screwshaft having a thread spirally positioned about the screw shaft so as toform a plurality of flights. The screw has three zones: a feeding zone,a compression zone and a metering zone, and in use is mounted in ahollow cylindrical barrel having a preferably smooth inner cylindricalwall which allows the screw to rotate within the hollow barrel.

As used herein, the term “feeding zone” refers to that zone of the screwwhere the material has not been melted. In the case of polymer pellets,for example, the pellets are present in their unmelted bulk form. Theterm “metering zone” refers to that zone of the screw where the materialhas been fully compressed. In the case of resin pellets, for example,the pellets are present in a completely molten form. The term“compression zone” refers to that zone of the screw where the materialis compressed. In the case of polymer pellets, for example, the pelletsare present in a mixed state between their bulk form and molten form.

A flight is characterized by its depth, which is defined as the heightof the flight above the screw shaft, by its width, and by its pitch,which is defined as flight length (the distance between two adjacentturns of the flight on the screw shaft) plus one flight width. If theflight has a pitch of 25 mm, it means that when the screw is rotatedonce, the polymer in the flight is moved axially 25 mm in the screw.

The present invention is based on the discovery that if the design ofthe flights is based upon the mass of the material to be present in theflights there is obtained a screw having a higher screw rotation speed,a higher throughput, and a decrease in injection molding cycle timecompared to conventional screws.

Thus, the invention relates to a screw comprising a screw shaft having athread spirally positioned about the screw shaft so as to form aplurality of flights, said screw having a feeding zone, a compressionzone and a metering zone. The screw has means formed in said screw forproviding a mass of material in a flight in the feeding zone that issubstantially the same as the mass of material in a flight in themetering zone, wherein said means includes flights formed in themetering zone having a pitch and depth based upon the volume of thematerial in a molten state and flights formed in the feeding zone havinga pitch and depth based upon the volume of the material in a bulk state.

Stated another way, in the present invention the depth, width and pitchof the flights of the screw are designed based upon the material to beused in the screw so that the absolute difference in the ratio of theactual volumetric flow to the theoretical volumetric drag flow ofmaterial in the feeding zone and the ratio of the actual volumetric flowto the theoretical volumetric drag flow of material in the metering zoneis less than 0.2, preferably less than 0.1, and more preferably lessthan 0.05.

This invention results in a screw which has a balanced mass flow alongthe screw, and thus a constant pressure gain along the screw withoutpressure peaks. In a preferred embodiment the ratio of the actualvolumetric flow to the theoretical volumetric drag flow of material inthe feeding zone and the ratio of the actual flow to the theoreticaldrag flow of material in the metering zone is from about 0.8 to 1.0.

The above ratios may be calculated on a volume per time basis.

The actual flow of material and the theoretical drag flow of material inthe feeding zone and the metering zone are determined as follows.

The actual flow of material in the metering zone is determined byweighing the output of material from the screw over a given period oftime. This mass flow rate may be converted into a volumetric flow rateby dividing the mass flow rate by the melt density of the material usedin the screw. By “melt density” is meant the density of the material,such as polymer, used in the screw when the material has been melted.

The mass flow rate of material in the screw is assumed to be constant,and the actual volumetric flow of material in the feeding zone isdetermined by taking the mass flow rate of material from the meteringzone and dividing that mass flow rate by the bulk density of thematerial used in the screw. By “bulk density” is meant the mass of thematerial, such as polymer particles or granules, used in the screwdivided by the total volume of the solid particles or granules and thevoids or open spaces between them.

By “drag flow” is meant the theoretical volumetric flow of materialwhich results from the relative movement between the screw and theinternal surface of the screw barrel, that is, the forward flow ofmaterial due to the turning of the helical screw which forces thematerial forward and through the barrel. Drag flow is proportional tothe product of the average relative velocity of the material and thechannel cross-sectional area of the cylindrical barrel. In other words,the drag flow is the volumetric pumping capacity of material, and isusually calculated on a volume per time basis. Drag flow is based on anumber factors related to the screw including the pitch, depth, widthand angle of the flights, and the speed of the screw. The drag flow,which is directed toward the outlet end of the screw, may be increasedby increasing the speed of the screw and/or by increasing the depth ofthe flights of the screw or by increasing the pitch of the flights ofthe screw.

The theoretical volumetric drag flow is calculated using well known,conventional formulae such as shown in Gerhard Schenkel,“Kunststoff-Extrudertechnik,” published by Carl Hanser Verlag, Munich(1963), pp. 123-125.

The theoretical volumetric drag flow calculated for the feeding zonemust be adjusted by a correction factor related to the geometry of theflights in that zone and the material used in the screw. This correctionfactor is necessary because of the bulk nature of the material in thefeeding zone and the influence of the flanks of the flights, and isnormally in the range of 0.7 to 0.95, more typically in the range of 0.8to 0.95. The correction factor may be obtained using known methods, suchas shown on page 123 of Schenkel where there is presented a graph of theratio of flight height to flight length to correction factor. Thecorrection factor is determined by taking the ratio of the flight heightto flight length and reading an appropriate correction factor from thechart.

While theoretically the calculation of the theoretical volumetric dragflow in the metering zone also needs to be adjusted by a correctionfactor, in fact the correction factor is very close to 1.0 because inthe metering zone the material is molten and because the ratio of theflight height to the flight pitch is very low, and thus this correctionfactor is approximated as 1.0.

A screw having the ratios described above has a relatively constantpressure gain per pitch along the screw. If pressure peaks occur in ascrew, stress will be applied to the material in the screw which willresult in screw deposit and a decrease in the mechanical properties ofthe material.

There is no limitation on the type of material that may be used in thescrew, although the screw has been found to be especially useful ininjection molding and extruding polymers. An example of a screw that hasthe desired difference in the ratio of the actual volumetric flow to thetheoretical volumetric drag flow of material in the feeding zone and theratio of the actual volumetric flow to the theoretical volumetric dragflow of material in the metering zone is a screw in which:

-   -   the pitch of at least a portion of the flights in the metering        zone is greater than the pitch of at least a portion of the        flights in the feeding zone;    -   the pitch of at least a portion of the flights in the feeding        zone is less than the outside diameter of the screw;    -   the pitch of at least a portion of the flights in the metering        zone is greater than the outside diameter of the screw;    -   the pitch of at least a portion of the flights increases through        the compression zone; and    -   the depth of at least a portion of the flights decreases through        the compression zone moving from nearer the feeding zone to        nearer the metering zone.

In a preferred embodiment, the geometry of the flights is such that thepitch of the flights in the metering zone is greater than the pitch ofthe flights in the feeding zone, the pitch of the flights in the feedingzone is less than the outside diameter of the screw, the pitch of theflights in the metering zone is greater than the outside diameter of thescrew, the pitch of the flights increases through the compression zone,and the depth of the flights decreases through the compression zonemoving from nearer the feeding zone to nearer the metering zone. As usedherein, the term “outside diameter of the screw” means the diameter asmeasured to include the screw shaft and the depth of the flight aboveand below the screw shaft.

The compression ratio of a screw quantifies the relative amount a screwcompresses a resin, and is based on the concept of dividing the volumeof a flight in the feeding zone by the volume of a flight in themetering zone. An approximation that is normally used as the compressionratio is the ratio of the depth of the flights in the feeding zone tothe depth of the flights in the metering zone.

Thus, the usual method for changing the compression ratio of a screw hasbeen to change the depth of the flights in the feeding and meteringzones. Since the depth of the flights in conventional screws is constantin the feeding zone and constant in the metering zone, the compressionratio of the screw would be increased by increasing the depth of theflights in the feeding zone, or decreasing the depth of the flights inthe metering zone, or doing both. However, if the compression ratio ofthe screw is too high it leads to the problems discussed above, namely,bridging and the build up of undesirable heat build up and screwdeposits.

The present invention is based on the discovery that one can obtain thebenefits of a high compression screw having a relatively high depth offlights in the feeding zone and relatively small depth of flights in themetering zone without the disadvantages associated with a highcompression screw, by providing a screw having an absolute difference inthe ratio of the actual volumetric flow to the theoretical volumetricdrag flow of material in the feeding zone and the ratio of the actualvolumetric flow to the theoretical volumetric drag flow of material inthe metering zone is less than 0.2, preferably less than 0.1, and morepreferably less than 0.05.

In effect, changing the pitch and the depth of the screw of theinvention, as described above, lowers the volumetric compression ratioof the screw substantially, and thereby removes the disadvantagesassociated with a high compression ratio screw. At the same time thescrew of the invention provides all the benefits associated with therelatively high feeding zone flight depths and relatively low meteringzone flight depths associated with a high compression ratio screw.

The compression ratio volume, calculated by taking the ratio of thefeeding zone volume to the compression zone volume, is not simple tomeasure when both the pitch and the depth of the flights of the screwchange. One reason is that the changing pitch causes a variation in theangle of the flights along the screw shaft. It has been discovered thatthe compression ratio volume for a screw having a changing flightpitches and changing flight depths may be approximated by taking theratio of the melt density to the bulk density for the polymer to be usedwith the screw.

The ratio of the melt density to the bulk density for many polymermaterials is approximately equal to 1.3, and this value of 1.3 is aminimum for the compression ratio of the screw. Below a ratio of 1.3,the polymer granules are not compressed enough to push entrapped air outof the polymer during the injection molding process.

With the present invention, improved results have been obtained with ascrew having very low compression ratio, i.e., equivalent to the lowerlimit of 1.3 or higher but lower than the compression ratio of a highcompression screw.

The discovery that a screw could be made and used successfully designedbased upon a small difference in the ratio of the actual volumetric flowto the theoretical volumetric drag flow of material in the feeding zoneand the ratio of the actual volumetric flow to the theoreticalvolumetric drag flow of material in the metering zone and with differentpitches in the feeding and metering zones, and a changing pitch in thecompression zone, was unexpected in view of the conventional teachingthat the screw designed based on the volume of material in the flightsand should have same pitch in each of the feeding, compression andmetering zones.

The invention also relates to a method for designing a screw such thateach flight of the screw has substantially the same mass of material,taking into account the physical state of the material in a flight,either as unmelted pellets or granules of material in the feeding zone,partially melted pellets or granules of material in the compressionzone, or completely melted material in the metering zone. To obtain thisconstant mass, the volume of each flight of the screw is designed insuch a way that the compression ratio in volume, that is, the ratio ofthe volume of a flight in the feeding zone to the volume of a flight inthe metering zone equals the ratio of the melt density/bulk density ofthe material to be used in the screw. It is preferred that thecompression ratio of the melt density/bulk density be increased up toabout 25%, preferably no more than 10%, to be sure that there is acontinuous homogenous feeding of the melt in the screw. The physicaldesign of the screw is then only a consequence of this method ofdesigning the screw.

The inventive method for designing the screw for use in injectionmolding or extrusion, which screw comprises a screw shaft having athread spirally positioned about the screw shaft so as to form aplurality of flights, said screw having a feeding zone, a compressionzone and a metering zone, may be characterized as follows. First, thematerial to be used in the screw is selected. Next, the diameter of thescrew to be used is selected. As used herein “diameter” includes thetotal distance as measured from the outer edge of the flights, and notjust the diameter of the screw shaft.

The depth and pitch of a flight in the metering zone are selectedbecause this depth and pitch, in combination with the diameter of thescrew, will determine the output capacity of the melt from the screw ata given turning speed of the screw. The depth of a flight in themetering zone is selected so as to provide a screw that, in operation,produces a molten material that has a homogeneous melt quality. One wayto estimate the pitch for a flight in the metering zone is the provide apitch that is from about 1 to 2 times the diameter of the screw.

The mass of material in a flight in the metering zone is determinedbased upon the volume of a flight in the metering zone and the meltdensity of the material. The volume of a flight needed in the feedingzone to provide the same mass of material as is present in a flight inthe metering zone is determined by taking the mass of material in aflight in the metering zone and then calculating the volume thatmaterial would have in its unmelted state using the bulk density of thematerial. As a starting point, one may estimate the depth of a flight inthe feeding zone to be 20% of the diameter of the screw. The depth andpitch of the flights in the feeding zone are adjusted so as to give acompression ratio for the screw which is about the same as the ratio ofthe melt density to the bulk density of the material. In order to assurea homogenous melt, this compression ratio may be increased up to about25%, preferably no more than 10%.

The depth and pitch of the flights in the compression zone of the screware determined so as to provide a mass of material in each flight of thescrew that is substantially constant. By “substantially constant” ismeant a mass of material that is up to about 110%, preferably no morethan 105%, of the mass of material in the flight of the metering zonedesigned above.

Stated another way, once the diameter of the screw and the material tobe used in the screw are selected, the depth and pitch of the flights inthe feeding zone, compression zone and metering zone are chosen so as toprovide an absolute difference in the ratio of the actual volumetricflow to the theoretical volumetric drag flow of material in the feedingzone and the ratio of the actual volumetric flow to the theoreticalvolumetric drag flow of material in the metering zone is less than 0.2,preferably less than 0.1.

Once the material and the screw diameter have been selected, one obtainsthe depth of a flight in the metering zone. Then, a rotating speed forthe screw is selected and the pitch of a flight in the metering zone isdetermined. As was stated above, one way to estimate the pitch for aflight in the metering zone is the provide a pitch that is from about 1to 2 times the diameter of the screw. With these factors determined thetheoretical volumetric drag flow of material in the metering zone may bedetermined.

The theoretical volumetric drag flow of the material in the feeding zoneis determined by multiplying the theoretical volumetric drag flow ofmaterial in the metering zone by the compression ratio of the screw,that is, by the ratio of melt density to bulk density of the material tobe used in the screw. As stated above, the theoretical volumetric dragflow of the material in the feeding zone may be increased up to about25%, preferably no more than 10%, to provide for a slightly higherpressure to be sure of a constant output of material from the screw.

The depth and pitch of a flight in the feeding zone is then determined.A starting point for selecting the depth of the flight is to use 20% ofthe diameter of the screw. The depth and pitch of the remaining flightsof the screw are determined so as to provide a substantially constantmass of material in each flight of the screw.

The output of the screw is determined and the actual flow of materialfrom the screw may be measured or calculated. Thus, one may determinethe absolute difference in the ratio of the actual volumetric flow tothe theoretical volumetric drag flow of material in the feeding zone andthe ratio of the actual volumetric flow to the theoretical volumetricdrag flow of material in the metering zone. A screw designed accordingto this method an absolute difference in the ratio of the actualvolumetric flow to the theoretical volumetric drag flow of material inthe feeding zone and the ratio of the actual volumetric flow to thetheoretical volumetric drag flow of material in the metering zone isless than 0.2, preferably less than 0.1.

The features of the screw of the present invention allow the screw tohave a higher screw rotation speed, a higher throughput, and a decreasein injection molding cycle time compared to conventional screws. Theinvention is illustrated in FIG. 2 where there is shown a screw 27having a feeding zone 29, a compression zone 31 and a metering zone 33.Screw 27 is housed in a hollow cylindrical barrel 35 having asubstantially constant inner diameter. Polymer resin, which may be inany convenient form, such as pellets, granules, flakes or powder, is fedthrough opening 37 in barrel 35 into feeding zone 29 where screw 27turns to pack and then push the pellets into compression zone 31 as witha conventional screw.

Screw 27 has a screw shaft 39 and a thread 41 spirally positioned aboutshaft 39 to form feeding zone flights 43, compression zone flights 45and metering zone flights 47.

The pitch of feeding zone flights 43 is smaller than the outsidediameter of screw 27, and, in a preferred embodiment, the pitch of eachof feeding zone flights 43 is approximately equal. The pitch of meteringzone flights 47 is larger than the outside diameter of screw 27, and, ina preferred embodiment, the pitch of each of metering zone flights 47 isalso approximately equal. Further, the pitch of feeding zone flights 43is smaller than the pitch of metering zone flights 47.

As is shown in FIG. 2, the depth of compression zone flights 45gradually decreases moving from nearer feeding zone 29 towards meteringzone 33, and the pitch of compression zone flights 45 graduallyincreases moving from nearer feeding zone 29 towards metering zone 33.The change in depth of compression zone flights 45 is obtained becausein compression zone 31 screw shaft 39 has the shape of a tapered cone.While the depth of compression zone flights 45 decreases while movingfrom nearer feeding zone 29 towards metering zone 33, it is notnecessary that the depth of each successive compression zone flight 45be smaller than the previous one. Similarly, while the pitch ofcompression zone flights 45 increases from nearer feeding zone 29towards metering zone 33, it is not necessary that the pitch of eachsuccessive compression zone flights 45 be larger than the previous one.

The inventive screw may be used in an injection molding machine, or anextruder, or it may be used as the melting section of a larger screw.

While the invention has been illustrated as having one flight, as isknown to those skilled in the art, the scope of the present inventionincludes a screw having more than one flight.

EXAMPLES Example 1 and Comparative Example 2

In Example 1 a screw according to the invention was made and inComparative Example 2 a conventional screw was made. The physicaldimensions of the screws are set forth in Table 1 below.

Delrin® 500 P, a polyacetal resin available from E. I. du Pont deNemours and Company (DuPont), was injection molded using both screws.The resin had a ratio of melt density/bulk density of 1.16/0.87=1.33.The results are summarized in Table 1 below. TABLE 1 Comparative Example1 Example 2 Screw diameter, mm 30 30 Depth of flights in the feedingzone, mm 8 7 Pitch of flights in the feeding zone, mm 28 30 Depth offlights in the metering zone, mm 2.3 2.2 Pitch of flights in themetering zone, mm 50 30 Screw speed (rpm) 250 125 Screw output (kg/hr)61 25 Compression ratio (volume) 1.47 2.6 Feeding zone actual flow,liters/hr 72 29 Feeding zone drag flow correction factor 0.81 — Feedingzone theoretical drag flow, liters/hr 73 37 Ratio of feeding zone actualflow to theoretical 0.98 0.78 drag flow Metering zone actual flow,liters/hr 54 22 Metering zone theoretical drag flow, liters/hr 54 17Ratio of metering zone actual flow to 1.00 1.25 theoretical drag flowDifference in ratio of actual flow to theoretical 0.02 0.47 drag flow infeeding zone and metering zone

The screw of Example 1 had a small difference in the ratio of the actualflow to the theoretical drag flow of material in the feeding zone andthe ratio of the actual flow to the theoretical drag flow of material inthe metering zone compared to the screw of Comparative Example 2. Thus,the screw of Example 1 produced a homogenous melt, a more consistentscrew retraction time and allowed a higher RPM, that is, a higher outputof resin that the screw of Comparative Example 2 without creating screwdeposit, splays, bridging, or other defects.

Example 3 and Comparative Example 4

Zytel® 135 F, a nylon resin available from DuPont, was also injectionmolded as in the previous Examples. The resin had a ratio of meltdensity to bulk density of 0.94/0.67=1.40. In Example 3, the resin wasinjection molded using a screw according to the invention and in Example4, the resin was injection molded using a screw according to the priorart. The results are summarized in Table 2 below. TABLE 2 ComparativeExample 3 Example 4 Screw diameter, mm 32 32 Depth of flights in thefeeding zone, mm 8 5.9 Pitch of flights in the feeding zone, mm 26 32Depth of flights in the metering zone, mm 2.1 2.1 Pitch of flights inthe metering zone, mm 48 32 Screw speed (rpm) 275 300 Screw output(kg/hr) 40 33 Compression ratio (volume) 1.56 2.4 Feeding zone actualflow, liters/hr 59 48 Feeding zone drag flow correction factor 0.80 —Feeding zone theoretical drag flow, liters/hr 81 76 Ratio of feedingzone actual flow to 0.73 0.64 theoretical drag flow Metering zone actualflow, liters/hr 42 35 Metering zone theoretical drag flow, liters/hr 5836 Ratio of metering zone actual flow to 0.73 0.95 theoretical drag flowDifference in ratio of actual flow to 0 0.31 theoretical drag flow infeeding zone and metering zone

The screw of Example 3 had no difference in the ratio of the actual flowto the theoretical drag flow of material in the feeding zone and theratio of the actual flow to the theoretical drag flow of material in themetering zone compared to a difference of 0.31 for the screw ofComparative Example 4. Thus, the screw of Example 3 produced ahomogenous melt, a more consistent screw retraction time and allowed ahigher RPM, that is, a higher output of resin that the screw ofComparative Example 4 without creating screw deposit, splays, bridging,or other defects.

Example 5 and Comparative Example 6

Delrin®500 P is injection molded as in the previous Examples using ascrew having a diameter of 65 mm. In Example 3, the resin is injectionmolded using a screw according to the invention and in Example 4, theresin is injection molded using a screw according to the prior art. Theresults are summarized in Table 3 below. TABLE 3 Comparative Example 5Example 6 Screw diameter, mm 65 65 Depth of flights in the feeding zone,mm 10 7.8 Pitch of flights in the feeding zone, mm 40 65 Depth offlights in the metering zone, mm 2.7 2.8 Pitch of flights in themetering zone, mm 75 65 Screw speed (rpm) 180 140 Screw output (kg/hr)185 110 Compression ratio (volume) 1.6 2.5 Feeding zone actual flow,liters/hr 212 127 Feeding zone drag flow correction factor 0.84 —Feeding zone theoretical drag flow, liters/hr 248 279 Ratio of feedingzone actual flow to theoretical 0.85 0.46 drag flow Metering zone actualflow, liters/hr 159 95 Metering zone theoretical drag flow, liters/hr170 121 Ratio of metering zone actual flow to 0.93 0.79 theoretical dragflow Difference in ratio of actual flow to theoretical 0.08 0.33 dragflow in feeding zone and metering zoneThe screw of Example 5 has a small difference in the ratio of the actualflow to the theoretical drag flow of material in the feeding zone andthe ratio of the actual flow to the theoretical drag flow of material inthe metering zone compared to a difference of 0.33 for the screw ofComparative Example 6. Thus, the screw of Example 5 produces ahomogenous melt, a more consistent screw retraction time and allowed ahigher RPM, that is, a higher output of resin that the screw ofComparative Example 6 without creating screw deposit, splays, bridging,or other defects.

1-17. (canceled)
 18. A method of injection molding or extruding apolymer composition using an apparatus that includes a screw adapted forbeing housed in a cylindrical barrel having a substantially constantinner diameter, said polymer composition having a predetermined bulkdensity associated with particles, granules or pellets of saidcomposition and a predetermined melt density when said polymercomposition is fully melted and compressed, said screw comprising: ascrew shaft defining a longitudinal axis and having a thread spirallypositioned about the longitudinal axis of the shaft; said spirallypositioned thread defining a substantially constant outside diameter ofthe screw that is less than the inner diameter of the cylindricalbarrel; said spirally positioned thread defining a pitch along saidscrew shaft; said screw shaft defining a root diameter that is less thanthe outside diameter of the screw; said screw defining a feeding zone, acompression zone and a metering zone along its longitudinal axis; saidpitch of said spirally positioned thread and said root diameter of saidscrew shaft in said feeding zone and said metering zone defining avolume compression ratio; said method comprising the steps of: providinga screw wherein said pitch of said spirally positioned thread and saidroot diameter of said screw shaft in said feeding zone and said meteringzone cause said volume compression ratio to be greater than or equal tothe ratio of the predetermined melt density to the predetermined bulkdensity of the polymer composition and up to 1.25× the ratio of thepredetermined melt density to the predetermined bulk density of thepolymer composition; feeding particles, granules or pellets of saidpolymer composition into said feeding zone; heating said polymercomposition to a temperature sufficient to cause said particles,granules or pellets to melt; rotating said screw with respect to saidcylindrical barrel so as to cause said particles, granules or pellets tocompress in said compression zone while melting and forcing said fullymelted and compressed polymer composition to exit the metering zone. 19.A method in accordance with claim 18 wherein said volume compressionratio is greater than or equal to the ratio of the predetermined meltdensity to the predetermined bulk density of the polymer composition andup to 1.10× the ratio of the predetermined melt density to thepredetermined bulk density of the polymer composition.
 20. A method inaccordance with claim 18 wherein said volume compression ratio is equalto the ratio of the predetermined melt density to the predetermined bulkdensity of the polymer composition.
 21. A method in accordance withclaim 18 wherein the ratio of the predetermined melt density to thepredetermined bulk density of the polymer composition is approximately1.3.
 22. A method in accordance with claim 18 wherein the ratio of thepredetermined melt density to the predetermined bulk density of thepolymer composition is approximately 1.4.